Field of the Invention
The present invention relates generally to nuclear technologies. More specifically, particular embodiments of the invention claimed herein relates to nuclear fuels and related methods for use in various types of nuclear reactors.
Description of Related Art
Nuclear fuel is what is “consumed” by fission to produce energy in a nuclear reactor. Nuclear fuels are very high-density energy sources and it is clear from the initial analysis of the March 2011 Fukushima accident that the failure of the nuclear fuel after shutdown has been the most important cause of the damage to the reactors and the environment. The immediate reason for the fuel failure was the lack of adequate cooling for the decay heat generated after reactor shutdown. The fact that the fuel failed rather rapidly and uncontrollably soon after cooling was compromised, however, points to substantial inherent weaknesses in this component of the nuclear reactor.
Oxide fuels such as uranium dioxide are commonly used in today's reactors because they are relatively simple and inexpensive to manufacture and can achieve very high effective uranium densities, have a high melting point and are inert to air. They also provide well-established pathways to reprocessing. The thermal conductivity of these fuels, however, is very low and goes down as the temperature goes up. The low thermal conductivity can lead to overheating of the center part of the pellets during use and difficulty in heat dissipation during loss of coolant events.
Virtually all fuel used in light water reactors (LWRs) is uranium dioxide (UO2). The uranium dioxide powder is compacted into cylindrical pellets and sintered at high temperatures to produce ceramic nuclear fuel pellets with a high density. Such fuel pellets are then stacked into metallic tubes (cladding). Cladding prevents radioactive fission fragments from escaping from the fuel into the coolant and contaminating it. The metal used for the tubes depends on the design of the reactor. Stainless steel was used in the past, but most reactors now use a zirconium alloy which, in addition to being highly corrosion-resistant, has low neutron absorption. The use of zirconium instead of stainless steel allows lower enrichment fuel to be used for similar operating cycles. Zirconium, however, is much more prone to react with steam to produce hydrogen at high temperatures.
Recently, micro-encapsulated tristructural-isotropic (TRISO) fuel particles compacted within a graphite matrix have been proposed for the next generation gas-cooled reactors. A TRISO fuel particle comprises a kernel of fissile/fertile material coated with several isotropic layers of pyrolytic carbon (PyC) and silicon carbide (SiC). These TRISO particles are combined with a graphite matrix material and pressed into a specific shape. While the TRISO fuel forms offer better fission product retention at higher temperatures and burnups than metallic fuel forms, they also provide only one containment shell (i.e., SiC layer) against fission product release to the coolant, and some fission products may migrate out of the kernel and through the outer layers and escape into the graphite matrix and coolant.
The sealed tubes containing the fuel pellets are called fuel rods. The fuel rods are grouped into fuel assemblies that are used to build up the core of a power reactor.
The fuel assemblies consist of fuel rods bundled in arrangements of 14×14 to 17×17 depending on the core design. One type of fuel is known as pressurized water reactor fuel, or PWR. PWR bundles are about 4 meters in length. In PWRs, control rods are inserted through the top directly into the fuel bundle.
Another type of fuel is known as boiling water reactor fuel, or BWR. The fuel assemblies in BWRs are “canned” within a thin tube surrounding each bundle. As the water physically changes phase and boils as it moves up through the BWR assemblies, the canned arrangement is adopted to prevent local density variations from affecting neutronics and thermal hydraulics of the overall reactor. There are typically 91 to 96 fuel rods per assembly and 400-800 assemblies in the reactor core. Control rods are inserted from the bottom as cruciform blades surrounding the canned assemblies.
Nuclear fuel, like any material in a high-radiation environment, can undergo substantial changes in its properties during reactor operations. Moreover, the occurrence of nuclear reactions will cause significant changes in the fuel stoichiometry over time, leading to cracking and fission gas release. As the fuel is degraded and cracks, the more volatile fission products trapped within the uranium dioxide may become free to move into the fuel-clad gap. As the fuel pin is sealed, the pressure of the gas filling the gap will increase and it is possible to deform and burst the cladding. The swelling of the fuel can also impose mechanical stresses on the cladding.
Once the geometry of the fuel rod is changed by excessive swelling, its heat transfer behavior may be degraded, with significant increase in the temperature of the cladding possible. The common failure process of fuel in the water-cooled reactors is a transition to film boiling and subsequent ignition of zirconium cladding in the steam. In a loss-of-coolant accident (LOCA) the surface of the cladding could reach a temperature between 800 and 1400° K, and the cladding will be exposed to steam for some time before water is re-introduced into the reactor to cool the fuel. During this time when the hot cladding is exposed to steam, some oxidation of the zirconium will occur to form a zirconium oxide and produce hydrogen. The oxidation can produce breaching of the fuel clad and subsequent release of the radioactive fission products.
The vast majority of nuclear fuels used today consist of uranium dioxide (UO2) pellets stacked inside a sealed cladding tube of zirconium alloy to make a fuel rod. Such fuels have three main weaknesses, however: (1) the presence of large amounts of zirconium in the clad that can react with steam at high temperature to produce hydrogen, (2) the fact that the fission products are only loosely bound to the fuel after they are produced, and (3) the very low conductivity of the fuel itself, which causes very high temperatures in the fuel and impedes the cooling of the fuel during off-normal situations. A fuel clad with a less reactive metal (like stainless steel) or a non-metal, having a higher conductivity (like a carbide or nitride fuel) and tightly bound fission products, would not have produced the large amounts of hydrogen responsible for the explosions at the Fukushima plant or the high temperatures responsible for the rapid failures after loss of cooling and the large releases of radioactivity that occurred after fuel failure.
It is clear that oxide fuels in zirconium cladding, the form most commonly used in LWRs, are vulnerable to LOCA conditions and can fail in catastrophic ways, due to (1) the adverse combination of a chemically active cladding, (2) loosely bound fission products, and (3) poor heat transfer capabilities. Thus, there exists a need for alternatives to oxide fuels that can be used to mitigate these concerns.